Bearings
A ball bearing reduces friction for axial movement. Ball bearings cost little and are widely available, mostly from the skateboard industry. They usually come sealed and pre-lubricated, so you do not need to add lubrication.
Types
| Abbrev. | Type |
|---|---|
| Z | Shielded, single |
| ZZ | Shielded, double |
| RS | Sealed, single |
| 2RS | Sealed, double |
| V | Non-contact seal, single |
| VV | Non-contact seal, double |
| DDU | Contact seals, double |
| NR | Snap ring and groove |
| M | Brass cage |
Sizes
| Abbrev. | Inner D. | Outer D. | Thickness |
|---|---|---|---|
| 608 | 8 mm | 22 mm | 7 mm |
| 688 | 8 mm | 16 mm | 5 mm |
| 686 | 6 mm | 13 mm | 5 mm |
| 626 | 6 mm | 19 mm | 6 mm |
| 625 | 5 mm | 16 mm | 5 mm |
| 624 | 4 mm | 13 mm | 5 mm |
| 623 | 3 mm | 10 mm | 4 mm |
| 603 | 3 mm | 9 mm | 5 mm |
| 633 | 3 mm | 13 mm | 5 mm |
The 608 size ball bearings in particular are extremely low cost. Their 8 mm bore size almost forces us to mount them on:
M8 threaded rod or M8 bolt, as in the Mendel Z-axis; or 5/16 inch bolts or 5/16 inch threaded rod, as in the SAE Prusa Mendel; or something like a plastic bearing clip 01 (still experimental).
They will need shims to fit snugly on threaded rod. Thin aluminum (pop cans) often works.
The 626 size ball bearing have a 6 mm bore size. We can mount them on:
M6 bolts and M6 threaded bar, as in the RepRapPro Huxley frame assembly.
The 624 bearing have a 4 mm bore size. We can mount them on:
M4 bolts, as in the Mendel X-axis; or #8 bolts (4.1656 mm) with a little tweaking, as in the SAE Mendel.
The 626 and 624 bearings seem to cost more than 608 bearings, but M6 and M4 bolts and threaded bar cost less than M8 threaded bar and bolts, so it's almost a wash.
ABEC Scale
The ABEC scale sets industry standards for ball-bearing tolerances. Manufacturers use these specifications to produce precision bearings in designated classes. Equipment manufacturers also check these dimensions to design parts that fit.
High ABEC-rated bearings serve precision applications like aircraft instruments and surgical equipment. Lower-rated bearings suit most other uses, including vehicles, hobbies, and industrial machinery. High ABEC bearings deliver optimal performance for applications that demand high rotational speed and smooth operation.
Tolerance classes
There are five classes: 1 (widest tolerance), 3, 5, 7, 9 (tightest tolerance)
The naming scheme reverses the expected order: higher ABEC numbers mean tighter tolerances. Higher ABEC classes offer better precision and efficiency, plus greater speed potential, but they do not guarantee faster spinning on their own.
Bearings that meet the least strict class (ABEC 1) qualify as "precision" bearings. Bearings below ABEC 1 cannot claim precision because their tolerances are too loose.
Correspondence with ISO 492
The International Organization for Standardization (ISO) equivalent standard is ISO 492.[3][4] The German equivalent is DIN 620. The Japanese equivalent is JIS B1514.[5]
| ABEC | ISO 492 | DIN 620 | JIS B1514 |
|---|---|---|---|
| ABEC 1 | normal / class 6X | P0 | Class 0 |
| ABEC 3 | class 6 | P6 | Class 6 |
| ABEC 5 | class 5 | P5 | Class 5 |
| ABEC 7 | class 4 | P4 | Class 4 |
| ABEC 9 | class 2 | P2 | Class 2 |
Ball materials
Commonly used ceramics include Si3N4 (Silicon Nitride), ZrO2 (Zirconium Dioxide) and SiC (Silicon Carbide), with Silicon Nitride being the hardest and most practical in skateboarding.
TLDR; Use steel when load is changing (tricks, park, etc). Use ceramic when speed is needed (racing, slalom, etc).
Retainers
Used to evenly space the balls within a bearing.
Related
- Lubrication - Bearing lubrication and seal types
- Tribology - Rolling contact friction and wear
- Journals - Sliding-surface bearing counterpart
- Power Transmission - Shaft power transfer components
- Gears - Paired rotating machine elements
- Material Selection - Steel vs ceramic ball materials
- Fasteners - Bolts and rod for bearing mounting
- Fatigue Fracture - Bearing fatigue failure modes